Issues with joining 3D scanned OBJs
Here is a procedure for baking the color from a 3D scan to a UV map in Blender: https://software.intel.com/en-us/blogs/2015/03/24/RealSense-3D-scan-Vertex-Color-to-UV-Map-Blender

Follow this process for both meshes (the head then the body). Then try to join them. If the textures are baked and applied via a UV map, both colors should remain when you join the meshes.